Customizable default search fields

I would love to be able to have the list of searchable fields _search_fields = ('artist', 'title', 'comments', album', 'albumartist', 'genre') be customizable, as discussed with Adrian here:

https://groups.google.com/forum/#!topic/beets-users/9Os4LkNq484

While the above would make a fine default, some of us want more power over what gets found in a simple query.

Thank you!!

Start by locating the `_search_fields` list and the simple-query search path in beets. Review the linked beets-users discussion for the intended configuration behavior, then verify that a configured field list replaces or extends the default fields used by simple queries.

Written by the indexing model from the issue text.